1961 Fender Bassman 6G6 Bass Channel

[Steelwitch]

Here’s a link to the schematic: https://images.app.goo.gl/mMEAbXUnVScHQXwu8

There are ceramic disc caps in there, one of them was badly noisy and was replaced, besides freeze spray, none of the other suggestions bore any results. So from everything I know, I can confidently say it’s not a grounding issue. There is however about 6vDC leaking onto the treble pot and I can’t really track down where it’s coming from and the first preamp stage is very microphonic when tapping on the tube, maybe something to do with the first two stages being cascaded? The voltage looked higher than it says on the schematic. Says plate voltage should be 140 but it’s more in the range of 192

[nosaj]

Here’s a link to the schematic: https://images.app.goo.gl/mMEAbXUnVScHQXwu8

There are ceramic disc caps in there, one of them was badly noisy and was replaced, besides freeze spray, none of the other suggestions bore any results. So from everything I know, I can confidently say it’s not a grounding issue. There is however about 6vDC leaking onto the treble pot and I can’t really track down where it’s coming from and the first preamp stage is very microphonic when tapping on the tube, maybe something to do with the first two stages being cascaded? The voltage looked higher than it says on the schematic. Says plate voltage should be 140 but it’s more in the range of 192

I see 2 .1uf caps I'd investigate leading to the treble pot.

nosaj

[Steelwitch]

So, there is DC leaking through the .1 cap leading directly to the treble pot. 211v coming from the plate and about 4vdc across it. I changed it out but there’s still dc coming through. It has to be coming from something else, but what else could make it upstream to the bass preamp stage?

[nosaj]

So, there is DC leaking through the .1 cap leading directly to the treble pot. 211v coming from the plate and about 4vdc across it. I changed it out but there’s still dc coming through. It has to be coming from something else, but what else could make it upstream to the bass preamp stage?

Is there any DC on your chassis?

nosaj

[Helmholtz]

Is there any DC on your chassis?

nosaj

Did you mean "board", like with a conductive board?

[nosaj]

Did you mean "board", like with a conductive board?

Not exactly I was thinking a cap going to ground but those are jus for signal so never mind on that. But conductive board is a great possibility.

nosaj

[Steelwitch]

No DC on the chassis, but the board has about 20mv on it, I didn’t even consider a conductive board

[The Dude]

I've had similar issues when ground wires crack loose from the brass grounding plate. If it has one, give the ground wires that connect to the plate a little tug and make sure connections are solid.

[Steelwitch]

There were some spots on the board where it rose to over 100mv. Any tests I can do before confirming that this is the source?

[Steelwitch]

I've had similar issues when ground wires crack loose from the brass grounding plate. If it has one, give the ground wires that connect to the plate a little tug and make sure connections are solid.

All ground connections are definitely good! I have checked the grounding pretty thoroughly

[g1]

There were some spots on the board where it rose to over 100mv. Any tests I can do before confirming that this is the source?

You could try monitoring the DC on the pot while heating up the area with heatgun/hair-dryer. If it changes, it's likely stray resistance of the board. The heat may even fix it as well.

[Steelwitch]

You could try monitoring the DC on the pot while heating up the area with heatgun/hair-dryer. If it changes, it's likely stray resistance of the board. The heat may even fix it as well.

It does have an effect! Voltage started decreasing and then into negative, which I don’t fully understand. Could you explain this process in more detail? How much time with a heat gun should fix it and how long it safe to expose the board to heat like that

[Steelwitch]

Something else I just noticed is that the volume pot affects the amount of DC on the treble pot. I lifted the eyelet board from the insulating board to verify that it isn’t the insolation board— no real effect, and there’s no audible difference with a heat gun. I think it could still be a conductive board though, since there’s dc on the board itself

[Helmholtz]

Something else I just noticed is that the volume pot affects the amount of DC on the treble pot. I lifted the eyelet board from the insulating board to verify that it isn’t the insolation board— no real effect, and there’s no audible difference with a heat gun. I think it could still be a conductive board though, since there’s dc on the board itself

Did you really replace all of the Astron caps with good quality new ones?

[Steelwitch]

Did you really replace all of the Astron caps with good quality new ones?

Yeah I replaced the yellow .25 astrons with .22 Jupiters.. I could have sworn it was bad astrons, but it didn’t make a difference. I mean, once this issue is resolved I’m eager to see if the Jupiter’s are considerably better. Kinda sounds bad right now though